at91: create a legacy subtarget

In preparation for adding SAMA5D3 support, move the legacy ARMv5 based
platforms to a separate subtarget.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42408 3c298f89-4303-0410-b956-a3cf2f4a3e73